Exposure to carbon dioxide partial pressures exceeding about 7.6 mm Hg (millimeters of mercury, partial pressure of about 1%), for extended periods of time are known to cause health problems in humans and other mammals. As a result, in enclosed habitable environments such as those existing in a submarine, space craft or space suit, carbon dioxide partial pressures are typically maintained below about 1% via the use of solid regenerable and nonregenerable carbon dioxide sorbents such as soda lime, zeolite and carbon molecular sieves, solid oxides, alkali metal carbonates, alkali metal hydroxides, amines, and combinations thereof, among others.
In addition to these sorbents, amines such as monoethanolamine and diethanotamine are often used in a liquid phase to reduce carbon dioxide partial pressures via absorption. These amines are utilized in the aqueous phase, typically 25 wt. % (weight percent) to 30 wt. % amine. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced into the bottom of the tower. While intimately contacting the gaseous stream in a counter-current fashion, the amine solution chemically reacts with the carbon dioxide to absorb and remove the carbon dioxide from the gaseous stream. Desorption of the absorbed carbon dioxide then proceeds via a thermal regeneration process at temperatures in excess of about 150.degree. F. (about 66.degree. C.). During desorption, carbon dioxide and water evolve from the amine solution and are separated by condensing the water vapor in a heat exchanger. Once regenerated, the amine solution is recycled back to the absorption tower for additional carbon dioxide absorption.
Although amine solutions are capable of initially removing carbon dioxide to partial pressures below about 1%, they have a limited life due to degradation through oxidation of the amine. Oxygen present in the gas stream oxidizes the amine particularly at the elevated temperatures during desorption when the amine solution is heated. This oxidation is believed to reduce the amount of amine primary and secondary functional groups available for carbon dioxide absorption. Consequently, the amine solution's useful life is limited to only about six months of continuous use.
In contrast, if desorption is accomplished at ambient temperatures, the useful life of the amine solution will be extended; however, performance will be limited by low desorption rates. Due to both energy requirements and oxidation related degradation, amine sorbents utilized in closed environment systems are often regenerated at approximately ambient temperatures for a fixed desorption time. At ambient temperatures, the desorption of carbon dioxide is often limiting. Under these conditions, the desorption process will typically have insufficient time to fully desorb the absorbed carbon dioxide. Consequently, a portion of the absorbed carbon dioxide remains in the sorbent after the desorption process is complete, thereby reducing the capacity of the sorbent to absorb additional carbon dioxide. As a result, a decreasing portion of the carbon dioxide sorbent is used throughout the absorption-desorption cyclical process.
Commonly assigned U.S. Pat. No. 5,376,614 discloses a solid aminepolyol sorbent comprising about 1 wt. % to about 25 wt. % amine, about 1 wt. % to about 25 wt. % polyol, and the remainder being a porous support which provides the amine with structural integrity and a high surface area for gas/solid contact. While the solid amine sorbent disclosed in this reference provides a number of advantages over the previously described solid and liquid sorbents, increased concentration of amine in the sorbent and improvements in cyclic CO.sub.2 removal capacity are desirable. In addition, formation of the sorbent is a fairly complex process requiring either pre-forming an amine/polyol solution and then impregnating the support with this solution, or impregnating the support with an amine solution followed by a second impregnation of the support with a polyol solution.
It is therefore an object of the invention to provide a solid regenerable carbon dioxide sorbent having an increased amine concentration.
It is a further object of the invention to provide such a sorbent having high carbon dioxide removal rates and high desorption rates at ambient temperatures.
It is a still further object of the invention to provide a process for forming such a sorbent.
It is yet another object of the invention to provide a continuous use, cyclic absorption/desorption system using such a sorbent that is not influenced by the presence of humidity by using such a sorbent.